The present invention relates to a drier for a cooling gas circulated in a rotating electric machine, and especially relates to a drier for drying cooling gas fed out from an electric rotating machine through a pipe line, dehumidifying the cooling gas so as to dry it by cooling it outside of the rotating electric machine, and returning the dried cooling gas to the rotating electric machine through another pipe line.
In a conventional drier using an external cooling type gas drying method, the dew point of the cooling gas provided in the rotating electric machine is kept below a surface temperature of an internal gas cooler installed in the rotating electric machine.
In an external cooling type gas drying equipment for a rotating electric machine, as described in Japanese Patent Laid-Open Application 2-85676 (1995), a gas refrigerator for circulating the coolant and in which a shell-tube cooler is separated from a refrigerating unit is generally provided.
In the conventional system described above, in order to keep the dew point of the cooling gas provided in the rotating electric machine under a surface temperature of an internal gas cooler installed in the rotating electric machine, the condenser in the external cooling type gas drying equipment is cooled below the needed temperature. In this manner, dew drops in the shell-tube cooler are frozen and the circulation of the cooling gas for the electric rotating machine is obstructed so that dehumidifying and drying of the cooling gas are reduced. Therefore, the temperature of the coolant is controlled so as not to be below the freezing point of the coolant.
However, in a gas-refrigerator in which the shell-tube cooler is separated from the refrigerating unit, the response of temperature control to the fluctuation of the load is not very good, and the setting range of the temperature control becomes wide. Therefore, the temperature of the shell-tube cooler is set near 10.degree. C., and there arises a problem in that it becomes impossible to make the dew point of the cooling gas in the rotating electric machine fully low.